This invention relates generally to the oxidative carbonylation of toluene, catalyzed by rhodium complexes in the presence of an oxidant, whereby para-toluic acid is produced at selectivities over 90%, or by iridium complexes wherein para-toluic acid is produced at somewhat lower selectivities.
The oxidative carbonylation reaction of aromatic compounds is an attractive method for the direct synthesis of aromatic carboxylic acids from arenes and carbon monoxide. In particular, oxidative carbonylation of toluene produces toluic acids that can be oxidized for manufacturing phthalic, isophthalic, and terephthalic acids employed as monomers for polyesters.
As used herein, xe2x80x9cturnover numberxe2x80x9d (TON) means the number of molecules transformed per catalyst molecule. Higher TON""s in comparable times indicate higher catalyst efficiency.
Palladium catalysts in the presence of oxidants (see, for example: Fujiwara, Y.; Kawata, I.; Sugimoto, H.; Taniguchi, H., J. Organomet. Chem. 1983, 256, C35; Jintoku, T.; Fujiwara, Y.; Kawata, I.; Kawauchi, T.; Taniguchi, H., J. Organomet. Chem. 1990, 385, 297; Ugo, R.; Chiesa, A., J. Chem. Soc., Perkin Trans. I 1987, 2625; Taniguchi, Y.; Yamaoka, Y.; Nakata, K.; Takaki, K.; Fujiwara, Y., Chem. Lett. 1995, 345) have been used, but the catalytic turnover numbers (TON) achieved for the carbonylation of toluene were low (2-8). Additionally, the para-selectivity of the reaction never exceeded 67%, normally being in the range of 40 to 55%. The TON""s were increased when oxygen was used as the oxidant in the presence of a cuprous promoter, but the para-selectivity was still relatively low (about 46%).
J. J. Van Venrooy, U.S. Pat. No. 4,093,647, reported carbonylation of toluene using palladium catalyst with thallium, which was added in stoichiometric amounts and improved para-selectivity.
Rhodium catalysts have also been used for the oxidative carbonylation of toluene, improving the para-selectivity to between 63 and 94% (see, for example: Kalinovskii, I. O.; Lescheva, A. A.; Kuteinikova, M. M.; Gel""bshtein, A. I. Zh. Obshch. Khim. 1990, 60, 123, J. Gen. Chem. USSR 1990, 60, 108 (English Translation)). Relatively good TON""s were obtained, but only when the reactions were run at 140xc2x0 C. using CO and O2 reactants with Cu promoter under pressures of about 150 to 250 psi and in ratios within explosive limits. F. J. Waller, U.S. Pat. Nos. 4,356,318; 4,416,801; 4,431,839; 4,463,103 to DuPont, and F. J. Waller, Catal. Rev. Sci. Eng. 1986, 28, 1, reported the carbonylation of toluene catalyzed with perfluorinated ion-exchange polymer/Rh composite occurring under more severe conditions, 150xc2x0 C. and 2000-4000 psi of CO containing 3% O2. Although the CO/O2 ratio was outside explosive limits, the TON""s achieved were low ( less than 9). Running the reaction in the presence of Cu2+, triflic acid and triflic anhydride resulted in slightly higher TON""s (42), the reaction conditions required still being relatively drastic.
The rhodium-catalyzed process described by Kalinovskii et al., Zh. Obshch. Khim. 1990, 60, 123, J. Gen. Chem. USSR 1990, 60, 108 (English Translation); Kalinovskii, I. O.; Lescheva, A. A.; Pogorelov, V. V.; Gel""bshtein, A. I., Khim. Tverd. Topliva 1993, 8, gives substantial quantities of hydroxylated side-products when water is present in the reaction mixture.
The present invention discloses a para-selective process for preparing toluic acid, comprising: combining toluene, carbon monoxide, having a pressure from about 0 and about 5000 psi, and an oxidant, with a rhodium catalyst, in an acid medium.
The present invention also discloses a para-selective process for preparing toluic acid, comprising: combining toluene, carbon monoxide, and an oxidant, with an iridium catalyst, in an acid medium.
Another disclosure of this invention is a process for preparing a mixture of p-toluic and m-toluic acids, comprising: combining toluene, carbon monoxide, and an oxidant, under oxidizing conditions, with a suitable rhodium or an iridium catalyst, in an acid medium, wherein the mixture of p-toluic and m-toluic acids may be oxidized to make terephthalic and isophthalic acids suitable for use, without separation, in the formation of polyester materials.
This invention relates to a process for the selective catalytic synthesis of para-toluic acid by reacting toluene in the presence of carbon monoxide (CO) and an oxidant in strong acid medium and in the presence of a rhodium catalyst. Examples of oxidants include, but are not limited to K2S2O8, oxygen and air, which by definition comprises oxygen. Although K2S2O8 is a preferred persulfate, other persulfates of the formula MxMxe2x80x2yS2O8.zH2O can be used. An example of strong acid medium includes, but is not limited to, trifluoroacetic acid. In the formula MxMxe2x80x2yS2O8.zH2O, M and Mxe2x80x2 are cations selected from the group consisting of Li, Na, K, Rb, Cs, and H, x+y=2, and z is any number from 0 to about 10. If K2S2O8 is used the process can be carried out at temperatures that are about or slightly above room temperature. The desired p-toluic acid is produced at greater than 90% selectivity. If oxygen gas (O2) or air is used the process can be carried out at temperatures up to about 120xc2x0 C. or above with CO/O2 or CO/air ratios below the explosive limit. The desired p-toluic acid is produced in about or above 65% selectivity. By either method, the p-toluic acid produced may subsequently be used to make terephthalic acid. The terephthalic acid can be used as a monomer in a variety of polymerization reactions, including the production of polyesters.
When O2 or air is used as the oxidant and the reaction is run at elevated temperatures for example, at about 120xc2x0 C. or above, significant amounts of m-toluic acid are formed in addition to the p-toluic acid. This mixture of p- and m-toluic acids may be used subsequently to make mixed terephthalic and isophthalic acids, which can be used for making polyester materials.
This invention also relates to a process for the selective catalytic synthesis of para-toluic acid by reacting toluene in the presence of carbon monoxide (CO) and an oxidant (for example, K2S2O8, oxygen or air) in mild acid (for example, acetic acid) medium in the presence of an iridium catalyst. If K2S2O8 is used the process can be can be carried out at temperatures at or above about 100xc2x0 C. When iridium catalyst is used the para-selectivity is generally less than about 80% selectivity, which is lower than that observed for rhodium catalyst. Any iridium compound capable of dissolving in the reaction medium under the reaction conditions described is suitable. Specific compounds are described below. Mixtures of p- and m-toluic acids may also be formed using these iridium catalysts, and, as described above, may be used to make mixed terephthalic and isophthalic acids, which in turn can be used for making polyester materials.
Reaction Media:
A mixture of trifluoroacetic acid and toluene is used as the reaction medium. Although other fluorinated carboxylic acids can be used (e.g., perfluorobutyric acid) trifluoroacetic acid was used mostly in the examples below. The trifluoroacetic acid:toluene ratio may vary in a broad range. Most experiments using K2S2O8 oxidant and rhodium catalyst were carried out in 1:1, volume/volume (v/v) trifluoroacetic acid:toluene. When oxygen gas or air is used as the oxidant with rhodium catalyst the trifluoroacetic acid:toluene ratio is smaller, from as little as about 1:100 up to about 1:5 (v/v), and the addition of trifluoroacetic anhydride results in improved yield of toluic acids.
When K2S2O8 is used as the oxidant, the reaction readily occurred in the presence of small amounts of water, although evidence has been obtained for the water gas shift reaction taking place when H2O is present. When water was present in small amounts no significant formation of cresols (side products) took place, in contrast with the substantial quantities of hydroxylated side-products observed when water was present in the rhodium-catalyzed process described by Kalinovskii, I. O.; Lescheva, A. A.; Pogorelov, V. V.; Gel""bshtein, A. I., Khim. Tverd. Topliva 1993, 8. We also found that using K2S2O8 as an oxidant and rhodium catalyst, very sluggish reactions with poor conversions and yields were observed in the presence of trifluoroacetic anhydride. This was unexpected since the beneficial effect of acid anhydrides (and trifluoroacetic anhydride in particular) had been reported by Kalinovskii et al., Zh. Obshch. Khim. 1990, 60, 123, J. Gen. Chem. USSR 1990, 60, 108 (English Translation).
Reaction Conditions:
When K2S2O8 or a related persulfate is used as the oxidant with rhodium catalyst, the most preferred carbon monoxide pressure is around 1 atm. (14 psig to 15 psig). The reaction can be performed under higher carbon monoxide pressure with some decrease in reaction rate, all other factors held constant. The reaction temperature can range from about 0xc2x0 C. to about 200xc2x0 C., preferably from about 20xc2x0 C. to about 100xc2x0 C., and most preferably from 20xc2x0 C. to 70xc2x0 C. It is preferred that the reaction mixture be stirred, most preferred the mixture be magnetically stirred at a rate of from 100 to 1000 rpm.
When air or oxygen is used as the oxidant with rhodium catalyst, the most preferred pressure of oxidant is 1000 psig, and the most preferred pressure of carbon monoxide is 120 psig. The reaction temperature in this case can range from about 0xc2x0 C. to about 300xc2x0 C., preferably from about 50xc2x0 C. to about 300xc2x0 C., and most preferably from about 100xc2x0 C. to about 200xc2x0 C. When K2S2O8 or a related persulfate is used as the oxidant with iridium catalyst, the most preferred carbon monoxide pressure is around 1 atm. (14 to 15 psig). The most preferred temperature range is from about 90xc2x0 C. to about 120xc2x0 C.
Other oxidants that are useful in this invention include hydrogen peroxide and trifluoroperacetic acid. Trifluoroperacetic acid is naturally generated in mixtures of hydrogen peroxide and trifluoroacetic acid or anhydride.
Catalysts:
Solutions of rhodium (III) oxide in trifluoroacetic acid are active catalysts. These solutions are obtained by dissolving commercially obtained rhodium (III) oxide in trifluoroacetic acid containing small amounts of water. This catalyst has not been previously reported for oxidative carbonylation reactions. Similar catalyst solutions are obtained by dissolving hydrated rhodium oxide, freshly precipitated from aqueous solutions of RhCl3 as in Example 4(a) below, in trifluoroacetic acid. Limited amounts of Cl (1 mole per mole Rh) do not deactivate the catalyst. Commercial samples of xe2x80x9cRhCl3.nH2Oxe2x80x9d are initially inactive but after a relatively long period of time (about 1 day at room temperature) under the reaction conditions (trifluoroacetic acid, toluene, CO, K2S2O8) catalytically active species are produced, which can successfully catalyze the reaction. Solutions of the carbonyl complex [(CO)2Rh(trifluoroacetate)]n also result in active catalysis under the reaction conditions, and solutions of the carbonyl complex [(CO)2Rh(trifluoroacetate)]n after treatment with oxidizing agents (for example hydrogen peroxide) also serve as active catalysts. Other carbonyl complexes [(CO)2RhX]n (X is an anionic ligand selected from the group consisting of acetylacetonate, trifluoroacetylacetonate, hexafluoroacetylacetonate, formate, acetate, benzoate, toluate, trifluoroacetate, nitrate, sulfate, phosphate, trifluoromethanesulfonate, carbonate, fluoride, chloride, bromide, iodide, methoxide, ethoxide, i-propoxide, n-propoxide, n-butoxide, sec-butoxide, and t-butoxide) also serve as catalyst precursors. Other rhodium complexes, for example (PPh3)3RhCl, may serve as catalyst precursors, although the phosphine ligand is not necessary for catalysis.
Iridium catalysts can also be used as stated above. The iridium catalyst is generated by a process which combines the reaction medium with an iridium compound from the group IrY3.nH2O where n is any number between 0 and about 10, and where Y is an anion selected from the group consisting of fluoride, chloride, bromide, and iodide. The iridium catalyst may also be generated by a process which combines the reaction medium with an iridium compound from the group IrYxe2x80x2(CO)yLz, where L is a neutral ligand selected from the group consisting of triphenylphosphine, pyridine, methylpyridine, aniline, and toluidine, and for y and z being any numbers such that y+z=2 or 3, and where Yxe2x80x2 is an anion from the group fluoride, chloride, bromide, iodide, formate, acetate, propionate, carbonate, nitrate, phosphate, sulfate, trifluoroacetate, cyclopentadienide, and pentamethylcyclopentadienide.
Oxidant:
Suitable oxidants include, but are not limited to oxygen gas, air, and peroxodisulfate, K2S2O8. When potassium K2S2O8, was used as the oxidant with rhodium catalyst the reaction occurred under very mild conditions (about 20xc2x0 C. to about 65xc2x0 C.). This oxidant has been previously used for the reported Pd-catalyzed oxidative carbonylation of aromatics with poor TON""s and selectivities. The use of this oxidant with Rh catalysts is novel.
When oxygen or air was used as the oxidant with rhodium catalyst the reaction required higher temperature to achieve significant rates, up to at least 200xc2x0 C. In contrast to previous reports (Kalinovskii et al., Zh. Obshch. Khim. 1990, 60, 123, J. Gen. Chem. USSR 1990, 60, 108 (English Translation); Kalinovskii, I. O.; Lescheva, A. A.; Pogorelov, V. V.; Gel""bshtein, A. I., Khim. Tverd Topliva 1993, 8) the process works with a CO/O2 ratio or CO/air ratio that is lower than the explosive limit.
Process Conditions and para-Selectivity:
Using K2S2O8 oxidant and rhodium catalyst, the catalytic reaction readily occurs at 1 atm. of CO and at 20xc2x0 C. to 65xc2x0 C. to produce toluic acid with 90-98% para-selectivity, the rest of the toluic acid being m-toluic acid. The highest selectivity was observed when the process was run at room temperature. The only side products detected by GC-MS were isomeric bitolyls (1-5% yield). In contrast, all previously reported Rh-catalyzed oxidative carbonylation reactions required temperatures above 140xc2x0 C. and PCo=200-4000 psi.